Cochlear response measurements in the ear canal pre-date the discovery of otoacoustic emissions. The importance of measurements of the acoustic impedance of the ear from a location in the ear canal has been long known, beginning with measurements obtained in 1928 and described in "Measurement of the Acoustical Impedances of Human Ears," by W. West, Post Office Electrical Engineers Journal, 21:293-300, 1928. Subsequent research has led to techniques that provide information on the external, middle and inner ear. One such system, described in U.S. Pat. No. 3,294,193, issued 1966 to Zwislocki, describes an instrument that measures a response function of the ear.
A subset of this response function provides measurements of reflected energy from the cochlea, because the mid-frequency part of the resistive component of the middle ear impedance is mainly due to the cochlear resistance, as first obtained by measurements in cats and rabbits, as described in "An Experimental Study of the Acoustic Impedance of the Middle Ear and its Transmission Properties," A. Moller, Acta Oto-Laryngol., 60:129-149, 1965, and Auditory Physiology, A. Moller, Academic Press, New York, 1983.
Early research had established the cochlear origin of subharmonic distortion products recorded in ear-canal pressure response measurements, as described in "On the Generation of Odd-Fractional Subharmonics," P. Dallos, J. Acousi. Soc. Am., 40:1382-1391, 1966, and in The Auditory Periphery, P. Dallos, Academic Press, U.S.A., 1973. Dallos also commented on his early research into the source of acoustic emissions in Comment on `Observations on the Generator Mechanism of Stimulus Frequency Acoustic Emissions-Two Tone Suppression` (D. T. Kemp and R. Chum) in Psychophysical, Physiological and Behavioral Studies in Hearing, page 42, E. deBoer and M. A. Viergever, editors, Delft University Press, 1980.
Following this early work by Dallos, otoacoustic emissions (OAEs) were discovered by D. Kemp, described in "Stimulated Acoustic Emissions From Within the Human Auditory System," D. T. Kemp, J. Acoust. Soc. Am., 64:1386-1391, 1978. Kemp's discoveries initiated an active period of research continuing to the present on cochlear-based signals that are inferred from pressure measurements in the ear canal. Broadly speaking, OAEs are classified into spontaneous otoacoustic emissions (SOAE), which refer to cochlear-based responses in the ear canal in the absence of any external stimulus, and evoked otoacoustic emissions (EOAE), which arise in response to an acoustic stimulus delivered into the ear canal.
These evoked responses are categorized according to the type of stimulus. The stimulus-frequency otoacoustic emission (SFOAE) is obtained using a sinusoidal signal, as described in "Observations on the Generator Mechanism of Stimulus Frequency Acoustic Emissions-Two Tone Suppression," D. T. Kemp and R. Chum, in Psychophysical, Physiological and Behavioral Studies in Hearing, pages 34-41, E. deBoer and M. A. Viergever, editors, Delft University Press, 1980. The SFOAE is a low-level signal measured in the ear canal at the frequency of the sine tone, which is based upon the property that the evoked emission has a saturating nonlinearity as the stimulus level is increased.
Other types of stimulus signals include a click-evoked otoacoustic emission (CEOAE) response, described in "Stimulated Acoustic Emissions From Within the Human Auditory System," J. Acoust. Soc. Am., 64:1386-1391, 1978, U.K. Provisional Patent No. 5467/78, 1978 to D. T. Kemp, U.S. Pat. No. 4,374,526, issued Feb. 22, 1983 to Kemp ("Kemp (1983)"), and U.S. Pat. No. 4,884,447, issued Dec. 1, 1989 to Kemp ("Kemp (1989)"). These references describe a measurement that is the pressure response to the presentation of a single click (also termed pulse), or the differential pressure response pulse to the presentation of clicks delivered at two (or more) intensity levels. A click, or, equivalently, a pulse, is a wide-bandwidth, deterministic, short-duration signal. The duration is usually limited by the duration of the impulse response of the acoustic source transducer, since the electrical input signal to the source transducer is typically much shorter than this impulse response duration. The stimulus duration is typically 1-4 milliseconds (msec), whereas the overall duration of the CEOAE response is in the range of 10-40 msec. In the prior art of Kemp (1983), the duration of the CEOAE response is assumed to extend over a 20 msec interval, and to prevent overlapping of the responses from succeeding pulse stimuli, it is stated that the time interval between pulses should be at least 20 msec, corresponding to a presentation rate of 50 Hz. Time gating of the response is recommended to remove the initial 5 msec of the total response, which is not included in the definition of the OAE response, and this initial portion of the response is thus excluded from the definition of the CEOAE response.
Distortion product otoacoustic emissions (DPOAEs or DPs) are OAEs measured in response to a stimulus comprised of two continuous, sinusoidal tones with frequencies .function..sub.1 and .function..sub.2. Information from the DPOAE includes the frequencies at which there are significant intensity levels, for example, at the 2.function..sub.1 -.function..sub.2 DP site. The term "site" refers to the location in the cochlea that is believed to generate the evoked emission. The underlying cochlear mechanisms leading to latencies or time delays in CEOAE also produce latencies in the DPOAE responses, which can be measured in terms of the group delay, as can be appreciated by those skilled in the art, and interpreted to provide information on cochlear micromechanics. The term "latency" refers to the time delay in the evoked response to the stimulus, whereas the group delay is defined based upon the rate of change of the signal phase with frequency. To control for probe nonlinearity, it is typical to use two source probes. One probe outputs the sine tone at .function..sub.1, and the other probe outputs the sine tone at .function..sub.2. The use of two independent probes controls for the intermodulation distortion that would otherwise occur at sum and difference frequencies if a single probe were used.
As described in "A Review of Otoacoustic Emissions." R. Probst, B. L. Lonsbury-Martin, and G. K. Martin, J. Acoust. Soc. Am. 89:2027-2067, 1991, current research indicates that these various OAE measurements systems are providing information from a common physiological origin, involving cochlear micromechanics and the mechanisms underlying signal transduction in the cochlea. Thus, advances in EOAE measurement techniques are aimed at providing more accurate, or more comprehensive, data regarding these cochlear mechanisms.
More complex stimuli have been used to measure EOAEs, including tone bursts created using short-duration, rectangular-windowed sinusoids (other windows including Gaussian have been used). Time averaging of the evoked response with artifact rejection and noise rejection is recommended by Kemp et al. (1986). "Artifact rejection" in this prior art has been taken to include a differential subtraction of responses, so as to eliminate the linear response, as in Kemp (1989), including the use of time gating. "Noise rejection" in this prior art is when the sound pressure level (SPL) exceeds some threshold during the portion of the signal after the first 5 ms. This is taken as evidence of a noise source external to the cochlear response. Time averaging is effective at attenuating noise, but has no benefits for reducing the nonlinear response output by the probe that is synchronous with the stimulus presentation rate. Artifact rejection, as used in the present application, differs in meaning from that of Kemp (1989), and is defined below. This probe nonlinearity interacts with the middle ear response in the first few milliseconds after presentation of a click stimulus. Both the middle ear response and the probe nonlinearity are large during this time interval.
Different linear cancellation procedures have been utilized in an attempt to control for this probe nonlinearity. One approach measures two responses, a first response to the stimulus at one level and a second response to the same stimulus, but at some higher level, typically on the order of 6 decibels (dB) higher. The low-level response is boosted or amplified by the same difference in level (6 dB in this example), and subtracted from the high-level response. If the system were linear, then the result would be a null response. Thus, the measured response after subtraction is due to the EOAE response and the synchronous residual probe nonlinearity. The residual probe nonlinearity can be significant and contaminates the beginning of the EOAE response. To eliminate this initial contamination, the EOAE response is typically nulled over the first 2-5 msec. However, eliminating the beginning of the EOAE response results in the elimination of the high-frequency content of the EOAE which is characterized by short latencies in this range. Presently, no techniques exist for CEOAE or tone-burst EOAE systems to control for probe nonlinearity that are as effective as the two-source probe technique for the DPOAE measurements.
Another standard subtraction technique in CEOAE systems, discussed in Kemp (1989), uses a single probe to produce four stimuli. The first three stimuli are three identical clicks and the fourth stimuli is a fourth click, but with the opposite polarity and three times the amplitude of the first three identical stimuli. The responses are summed to produce a response which would be zero if the cochlear EOAE behaved as an ideal linear response. This subtraction of the linear response is the "artifact rejection" of Kemp et al. (1986). This is argued as reducing random noise, but the probe nonlinearity is not controlled, and is worse than the previously described technique since the relative gain between high and low-level clicks is larger, thereby producing greater amounts of probe distortion. The initial 5 msec of the CEOAE response is nulled to remove probe artifacts.
Each of the existing EOAE systems lacks the ability to easily measure a wideband response or suffers from the inability to control for probe nonlinearities. Therefore, it can be appreciated that there is a significant need for a system and method to measure stimuli to produce evoked emissions while controlling for nonlinearities without the use of time-gating. Moreover, present theories do not allow the interpretation of click-evoked OAE responses within the framework of a distortion product-evoked OAE response model, and vice versa. The present invention provides this and other advantages, as will be seen from the following discussion and accompanying figures.